This invention relates in general to electrophotographic imaging members and, more specifically, to layered photoreceptor structures with an improved overcoat layer and processes for making and using the imaging members.
Electrophotographic imaging members, i.e. photoreceptors, typically include a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is an insulator in the dark so that electric charges are retained on its surface. Upon exposure to light, the charge is dissipated.
Many advanced imaging systems are based on the use of small diameter photoreceptor drums. The use of small diameter drums places a premium on photoreceptor life. A major factor limiting photoreceptor life in copiers and printers, is wear. The use of small diameter drum photoreceptors exacerbates the wear problem because, for example, 3 to 10 revolutions are required to image a single letter size page. Multiple revolutions of a small diameter drum photoreceptor to reproduce a single letter size page can require up to 1 million cycles from the photoreceptor drum to obtain 100,000 prints, a desirable goal for commercial systems.
For low volume copiers and printers, bias charging rolls (BCR) are desirable because little or no ozone is produced during image cycling. However, the micro corona generated by the BCR during charging, damages the photoreceptor, resulting in rapid wear of the imaging surface, e.g., the exposed surface of the charge transport layer. For example wear rates can be as high as about 16.mu. per 100,000 imaging cycles. Similar problems are encountered with bias transfer roll (BTR) systems. One approach to achieving longer photoreceptor drum life is to form a protective overcoat on the imaging surface, e.g. the charge transporting layer of a photoreceptor. This overcoat layer must satisfy many requirements, including transporting holes, resisting image deletion, resisting wear, avoidance of perturbation of underlying layers during coating.
Various overcoats employing alcohol soluble polyamides have been proposed in the prior art. One of the earliest ones is an overcoat comprising an alcohol soluble polyamide without any methyl methoxy groups (Elvamide) containing N,N'-diphenyl-N,N'-bis(3-hydroxyphenyl)-[1,1'-biphenyl]-4,4'-diamine. This overcoat is described in U.S. Pat. No. 5,368,967, the entire disclosure thereof being incorporated herein by reference. Although this overcoat had very low wear rates in machines employing corotrons for charging, the wear rates were higher in machines employing BCR. A cross linked polyamide overcoat overcame this shortcoming. This overcoat comprised a cross linked polyamide (e.g. Luckamide) containing N,N'-diphenyl-N,N'-bis(3-hydroxyphenyl)-[1,1'-biphenyl]-4,4'-diamine. In order to achieve cross linking of the polyamide polymer, Luckamide, having methyl methoxy groups was employed along with a catalyst such as oxalic acid. This tough overcoat is described in U.S. Pat. No. 5,702,854, the entire disclosure thereof being incorporated herein by reference. With this overcoat, very low wear rates were obtained in machines employing bias charging rolls (BCR) and Bias Transfer Rolls (BTR). Durable photoreceptor overcoatings containing cross linked polyamide (e.g. Luckamide) containing N,N'-diphenyl-N,N'-bis(3-hydroxyphenyl)-[1,1'-biphenyl]-4,4'-diamine (DHTBD) [Luckamide-DHTBD] have been prepared using oxalic acid and trioxane to improve photoreceptor life by at least a factor of 3 to 4. Such improvement in the bias charging roll (BCR) wear resistance involved crosslinking of Luckamide under heat treatment, e.g. 110.degree. C.-120.degree. C. for 30 minutes. However, adhesion of this overcoat to certain photoreceptor charge transport layers, containing certain polycarbonates (e.g., Z-type 300) and charge transport materials [e.g., bis-N,N-(3,4-dimethylphenyl)-N-(4-biphenyl )amine and N,N'-diphenyl-N,N'-bis(3-methylphenyl)-[1,1'-biphenyl]-4,4'-diamine] is greatly reduced under such drying conditions. On the other hand, under drying conditions of below about 110.degree. C., the overcoat adhesion to the charge transport layer was good, but the overcoat had a high rate of wear. Thus, there was an unacceptably small drying conditions window for the overcoat to achieve the targets of both adhesion and wear rate.